The Cies Islands archipelago was formed millions of years from the mid-Tertiary and Quaternary as a result of tectonic movements and the end of the last Ice Age that gave rise to the Galician.
The islands have an asymmetric profile with two strands of distinct morphological features. The western slope is steeper shore islands where sea erosion results in high cliffs and caves. The eastern side has a much smoother profile protected from the erosive action of wind and open sea, allowing the formation of beaches and dune systems.
Cies Islands have a climate with conditions similar to Mediterranean regions due to low annual rainfall (nearly half of which rains in Vigo). This feature along with the winds with high salt content that negatively affect the development of trees, makes the Cies Islands are a territory that differs markedly from the familiar territory of Galicia Atlantic.
In the Cies can see odd geological formations resulting from erosion. As honeycomb shapes (alveoli) and cacholas (taffoni) which are rounded cavities originating on the vertical surfaces of rocks as a result of a breakdown granular or flake.These forms can be seen in the stone of the bell.
If erosion is on horizontal surfaces formations are called sinks or pious (gnamma) and channels. These formations can be seen in the top of the prince.
Another interesting formation is between the North and Central beaches. The resort beach-dune-lagoon which connects the two islands. This system is closely linked to geomorphological winds and tidal currents and is a rich ecosystem of many marine species. The gap is formed between the dune system and artificial reef dam lake called children..